In printing devices, a media indexing system advances or indexes the media through the print zone during the printing process. Such systems can include motors, gears, rollers, sensors, and other components that work in conjunction to move the media incrementally. During each incremental movement of the media, a printing element, such as a print head, causes text and/or images to be applied to the media in rows or swaths.
However, manufacturing tolerances, assembly variations, and other imperfections in such indexing components can cause discrepancies between the desired or expected position of the media and the actual position which is realized. For example, one typical component in a media indexing system is a feedroll or feedroller which contacts the media and forces the media through the printing device. Manufacturing tolerances can cause the feedroller diameter to deviate from its ideal diameter and can also cause the feedroller circumference to deviate from its ideal circular shape. The deviation of the actual feedroller diameter from the ideal feedroller diameter can produce a cumulative feedrate error which corresponds to the average deviation in the desired media position during the movement. For example if the diameter is smaller than ideal, “runout” (feedrate) errors will occur causing the media to be underfed. In addition, a deviation of the actual feedroller shape from the ideal circular shape and/or a deviation of the axis of rotation from center, can produce irregular rotation and a substantially sinusoidal “once-per-revolution” (eccentricity) errors in the indexing of the media.
Other similar deviations in other indexing components can also introduce such errors in the indexing of the print receiving media. For example, eccentricity in the encoder wheel used to sense the feedroller position as well as various forces on the media being moved can cause the media to be advanced too little or too much. Such errors can vary within a printing device based upon the media type, environmental factors (such as temperature and humidity), and mechanism wear.
These media positioning errors can result in an undesirable appearance of the printed image. For example, indexing the media too far (i.e., overfeeding the media) might result in a gap or blank band between print swaths, while indexing the media too little (i.e., underfeeding the media) might result in overlapping print swaths that could create dark horizontal bands in the resulting image. Such swath misplacements can be particularly noticeable during particular printing modes or resolutions.
To compensate for such errors, media indexing calibration and compensation control systems and methods can be utilized. Such systems and methods can measure the system error and provide corresponding compensation to media indexing system components, to attempt to increase media positioning accuracy.
In many printing device media indexing systems where such compensation is utilized, encoders are utilized to sense the position of the feedroll or other rotational components within the system. An encoder includes a wheel having incrementally spaced marks which rotates with the rotational component being measured. An optical sensor detects the marks as they pass and the detection of each mark can be counted to keep track of the incremental movement of the wheel and, therefore, the movement of the component. However, in order to compensate for media indexing errors which are dependent upon the rotational position of the component, the encoder can be required to monitor the rotational or angular position of the component. To achieve this result, a home position mark or index mark could be provided on the encoder wheel, such that the encoder can sense when the wheel has reached a home position. However, some configurations of home position encoders can require increased manufacturing cost and space, while others can be error prone.
Accordingly, it is desirable to provide an improved printing device encoder with efficient and accurate home position locating capability. Such an improved encoder can be utilized in media positioning compensation or other rotational sensing needs within the printing device.